Development of Novel Carbohydrate-Based Anti-Bacterial Agents

Abstract

The World Health Organisation in 1997 recognised the need for the development of new drugs to combat the imminent threat of mass infection by multi-drug resistant strains of pathogenic micro-organisms that spread diseases such as tuberculosis (TB). With this in mind, the work covered in this thesis focused on the preparation of novel carbohydrate-based compounds as potential anti-bacterial agents, in particular targeted to mycobacterial infection. An essential cell wall structural component of various mycobacteria (including M. tuberculosis) is the galactofuranose (Galf) residue, which forms part of the polysaccharide connection between the mycolic acids and the peptidoglycan layer. Interference with the incorporation of Galf into this polysaccharide was expected to compromise the physical integrity of the bacterial cell wall leading to cell death. With minimal projected side-effects, owing to the lack of Galf residues in mammalian systems, hydrolytically-stable Galf analogues were selected as promising targets for the development of potential therapeutic agents against Galf containing pathogenic micro-organisms. In 2000, the von Itzstein group identified the novel Galf-based N,N-didecyl-S-(1-thio-?-D-galactofuranosyl) sulfenamide as a strong inhibitor of mycobacterial growth in vitro. As a result of this finding, a study of structure activity relationships (SAR) of galactofuranose derivatives as antibacterial agents has been undertaken. The primary focus has been on the synthesis of a family of glycofuranosyl derivatives based on the architecture of the N,N-dialkyl galactofuranosyl sulfenamide. To explore structure activity relationships, a number of related compounds were prepared. Chapter 2 describes the development of various synthetic routes towards a suite of S-, O-and N-galactofuranosides which were prepared from a common per-O-acylated Galf intermediate. The range of compounds included N,N-disubstituted sulfenamides and sulfonamides, branched and straight chain alkyl thioglycosides, thioureas and alkyl O-glycosides. In addition, the aglycon units incorporated were designed in order to identify the necessary characteristics for optimum anti-bacterial activity, and included aglycons both aromatic and aliphatic in character. For the purpose of investigating the requirement for a galactofuranose glycan moiety for anti-bacterial activity, the aglycons of the more active galactofuranosyl compounds (Chapter 2) were introduced onto alternate furanose sugars. Chapter 3 describes the synthetic strategies developed for the synthesis of gluco-and arabinofuranosyl compounds. Both glucofuranosyl sulfonamides and arabinofuranosyl sulfonamides and thioglycosides were prepared. Chapter 4 describes the synthesis of non-carbohydrate probes carrying the relevant aglycon units of the glycofuranosyl series. This series of compounds (containing either sulfenamide or sulfonamide functionality) were prepared to investigate the requirement of a carbohydrate moiety for anti-bacterial activity. Chapter 5 describes methods to improve the water solubility of the most active compounds prepared in Chapters 2 and 3. Methods explored included chemical modifications to the primary hydroxyl group of the sugar moiety, and experimentation with formulation of active compounds. Biological testing of the compounds prepared, carried out both in-house as part of the work for this thesis, and by various external testing facilities, is reported in Chapter 6. In assays against M. smegmatis the Galf 9-heptadecyl thioglycoside was found to be the most active compound with an in vitro potency of 1 µg/mL, comparable to the frontline anti-TB drug ethambutol (1-4 µg/mL). In addition, the thioglycoside displayed excellent broad spectrum Gram positive anti-bacterial activity in vitro, but was not active at concentrations below 64 µg/mL against a Gram negative organism. Experimental data supporting the information presented in Chapters 2 to 6 is detailed in Chapter 7. Selected proton NMR spectra are presented in Appendix 1, whilst Disc Diffusion and Zone assay data are tabulated in Appendix 2.